1. Technical Field
The present invention relates to a manufacturing method of an optical fiber base material that realizes high-speed manufacturing of the optical fiber base material, and also produces an optical fiber base material that has a uniform thickness in the longitudinal direction of a starting member.
2. Related Art
Conventionally, a variety of methods have been proposed for manufacturing an optical fiber base material. Among these methods, there is an outside vapor deposition (OVD) method for obtaining an optical fiber base material that includes rotating on its axis a starting member that is a core rod fused at both ends to dummy rods, moving the starting member and a plurality of burners back and forth relative to each other, depositing glass microparticles on the surface of the starting member, dehydrating the resulting material in an electric furnace, and performing sintering to obtain the optical fiber base material. This OVD method is commonly used because it is capable of mass-producing an optical fiber base material with a relatively desired refractive index distribution and a large opening diameter.
With this OVD technique, in order to improve the equipment productivity, ideas such as increasing the number of burners and increasing the deposition rate have been considered.
Patent Document 1 proposes a method that includes arranging burners along the entire region of the starting member that is to become the final product, and improving the deposition rate by partially traversing this portion of the starting member with the burners.
With this method, the number of burners can be significantly increased and the deposition rate can be improved, but since each burner traverses only a predetermined portion of the product region, there is a significant effect resulting from individual differences between the burners and variation in the flow rates and there is fluctuation in the deposition thickness of the glass microparticles at different longitudinal positions of the starting member, and therefore it is necessary to perform post-processing that includes an additional step of grinding the surface of the base material.
On the other hand, a method for improving the deposition rate by having each burner traverse the entire product region has been proposed that includes arranging a plurality of burners with movement axes along the starting member and around the circumference of the starting member.
With this method, since the burners traverse the entire product region, there is less likely to be fluctuation in the deposition thickness of the glass microparticles in the longitudinal direction due to variations in the gas flow rates or individual differences among the burners, but the burners must pass by each other during deposition, and the portions where the burners pass by each other experience a decrease in the deposition efficiency due to the interference between the flames of the burners and fluctuation in the deposition in the longitudinal direction.
To deal with this problem, Patent Document 2 proposes a method that includes increasing the speed at which the burners pass by each other, in order to decrease the effect of interference when the burners pass by each other.
However, with this method, the burners pass by each other at the same position every time the starting member is traversed, and therefore it is easy for fluctuation to occur between the positions where the burners pass by each other and positions where the burners do not pass by each other. In particular, current generation base materials have been becoming larger, and even when there is only a small fluctuation, this small fluctuation is amplified and results in a large fluctuation.
As another example, Patent Document 3 proposes a method that includes changing the movement range traversed by the burners for each movement axis, but with this method, there is an increase in the amount of non-product regions, which are the cone-shaped regions at the ends.
Patent Document 1: Japanese Patent No. 2809905
Patent Document 2: Japanese Patent No. 3581764
Patent Document 3: Japanese Patent No. 4690979
It is an objective of the present invention to provide an optical fiber base material manufacturing method that enables manufacturing of optical fiber base material at high speed and obtaining of a uniform deposition thickness in the longitudinal direction even when the burners are arranged with a plurality of movement axes.